Removing a heavy sash from a window is a two-person job, because the sash can weigh more than 100 pounds, as often occurs in schools, offices, and institutional buildings. The counterbalance systems for such heavy sash provide correspondingly large upward forces, and locking the shoes in place against the strong counterbalance forces requires more strength and security than is offered by shoe locking systems intended for smaller, residential window sash. Locking shoes for heavy sash have to be strong and rugged, and the security of their locked positions and their sash holding ability must be highly reliable, because of the danger and damage that could come from losing a sash support or accidentally releasing a locked shoe.
As a smaller residential sash is removed from a window, it is usually tilted from its vertical operating plane; and this transfers part of the sash weight from the support shoes to the person holding the sash. This is undesirable when the sash is especially heavy, because a person tilting a heavy sash might find she has more weight than she can handle safely. Also, shoes that accommodate sash tilting are generally more complex than shoes for a sash that remains vertical.
My invention solves these problems by providing strong locking shoes that reliably support and counterbalance a heavy sash and are also reliable at locking in place to allow the sash to be removed from the window. It involves security measures that ensure that the shoes are reliably locked before the sash is removed and that the sash is reliably replaced on its shoe platforms before the shoes are unlocked. It also ensures that the sash is removed from the window in a safe and reliable way and is guided back into operating position as it is replaced on the support shoes.